How Thick Should Insulation Be?

Insulation thickness matters because depth is how you build up R-value. Each material adds a certain amount of R-value per inch, so the deeper the layer, the more it resists heat flow — up to the point of diminishing returns.

That's why recommendations are usually given as a target R-value, with the inches following from whichever material you use.

Hitting a typical attic target often means roughly 10 to 18 inches of blown-in insulation, depending on the material and your climate. Colder regions push toward the higher end; milder climates can do well with less.

Fiberglass needs more depth to reach a given R-value than denser materials, while spray foam reaches it in far fewer inches — so 'how thick' always depends on what you're using.

Thickness only counts if the insulation is fluffed to its rated loft. Compressed insulation — squashed under stored boxes or packed too tightly into a cavity — loses R-value even though it looks like there's plenty of material.

That's a common reason an attic underperforms despite appearing full. Even, uncompressed coverage at the right depth is what delivers the rated performance.

The simplest way to know if you have enough is to measure the existing depth in several spots and compare it to the recommended R-value for your area. Thin or uneven coverage means there's room to improve.

A professional can measure accurately, account for the material type, and top up your attic to the right depth — along with sealing the air leaks that thickness alone won't fix.